Per the datasheet the Junction to Case (ground paddle) thermal resistance is 17.6°C / W on the HMC440QS16G for roughly 22°C temp rise; at 85°C this results in a 107°C for the junction temperature. Not quite sure why you need additional cooling at such a low Tj. We don't have data to the top of the plastic package if that's what you're looking for.

I'm in Central Apps, and this came in from a mechanical/thermal engineer from Raytheon,

a strategic customer for us in McKinney, TX. He replied:

"To answer your question "Not quite sure why you need additional cooling at such a low Tj",Current CCA thermal model with NO top cooling on this component predicts a board temperature of 106 C, giving a Tj of 128 C (106 + 1.25W x 17.6 C/W).This is for a chassis rail temp of 43 C, which is very low for our types of design in mil spec environments.

I have been asked by the project to determine how high the rail temperature can go before we exceed a Tj max, which for this component is 135 C.

So we can only increase the rail temperature by 7 degrees (135 - 128) if we don't add top cooling for this component.FYI there is an aluminum cover above this component that we can connect by thermal gap filler to the top of this HMC440QS16G.

But to determine how effective this approach will be I need to have the Theta (j to case-top) thermal resistance. If I try and "guesstimate" what it might be, I would calculate the thermal resistance from an internal die to the top surface of the plastic package, orR = L / k A = 0.03" / 0.0267 W/in C .091" .069" = 178.9 C/W (neglecting any heat spreading in the plastic)where L is the estimated thickness of the plastic above the die, k is the thermal conductivity of the plastic mold compound: 1.05 W/mK = .0267 W/in C,

A is the estimated area of the die, which I guessed as the size of the ground paddle.Can you tell me if I need to adjust any of these assumptions to better reflect the actual internal details?